76 ON THE rHYSIOGNOMT OF SERPENTS. 



malopsis, &c. The number, configuration, and disposition, 

 of these plates arc extremely variable ; and the enumera- 

 tion of these discrepancies, therefore, enter into the special 

 part of my work ; mo may, in the mean time, observe, 

 that those of the upper lip go on diminisliing towards the 

 end of the miizzle in most serpents, and the reverse in 

 some measure takes place in the lower jaw. It most ge- 

 nerally happens that the superior labials form the lower 

 border of the eye. 



This organ has its lateral borders furnished with small 

 plates, denominated the Oculars. Several Opliidians 

 have only a single anterior ocular, while the number 

 of the posterior oculars varies from two to four, according 

 to the species : sometimes they are prolonged below the 

 eye ; and in several serpents, whose heads are covered 

 with small scales, they are small, and form a range, sur- 

 rounding the whole perij)hery of the eye. In the TortrLx 

 scytale, all the marginal plates of the eye are joined in 

 one single piece, in the centre of which that organ is 

 placed. 



It rarely happens that the nostrils perforate the Nasal 

 plate, without dividing it vertically into two ; this, how- 

 ever, takes place in the genera Tortrix, Boa, Elaps, and 

 in most of the true venomous snakes ; sometimes even, 

 as in the Elaps, the nostrils open just between two plates 

 of different forms, the posterior of which may be consi- 

 dered as a Frenal. The form of the nasals, and their po- 

 sition, undergo considerable modifications in different 

 species of Ophidians : in at^uatic serpents, those plates 

 are usually near the summit of the muzzle, and some- 

 times serve the purpose of the superior frontals, as in the 

 Hydrophis ; but more usually they occupy the sides of 

 the muzzle, touching the rostral plate by their anterior 

 edges. 



These are most commonly followed by another plate, 

 the Frenal, which extends to the anterior oculars : this 

 plate is, however, wanting in a great number of Ophidi- 

 ans, while its place is occupied in others by two, three, or 

 more plates, of very diflferent extent and form. 



